Your physicist is correct.
Or at least he would be if your 'jet' was powered by a rocket engine!
But this doesn't explain the operation of high bypass turbo-fan engines, where a large majority of the thrust is produced by the 'fan' part. This is essentially a shrouded propellor, and has nothing whatsoever to do with the 'jet' part, other than receiving its power from the (typically 1st stage) turbine arangement.
I'd go along with your original definition:
when the gasses emerging from the engine are faster than the speed of the aircraft, forward thrust is created.
When air is accelerated backwards (as in your description), thrust is generated. This applies equally to the fan part, and the jet part - both accelerate air backwards, and both generate forward thrust.
I think care is needed to distinguish between a 'jet' engine as used by the majority of airliners, and 'rocket' engines, which your physicist might well have been referring to.
PM